This invention concerns flexible films made from certain polymers that possess two important characteristics. First, they are substantially halogen-free. In other words, current analytical techniques do not reveal the presence of detectable quantities of chemically combined halogen. Second, the polymers yield films that can be activated (bonded or sealed) with high frequency (HF) electromagnetic energy. More particularly, this invention concerns HF or radio frequency (RF) weldable films containing carbon monoxide (CO) copolymers or interpolymers.
Products manufactured from flexible polyvinyl chloride (f-PVC) have been used for many years in a multitude of applications. In recent years, however, growing concern about the environmental impact of halogen-containing polymers, from manufacture through disposition, has led to a desire to find alternatives for halopolymers, especially for PVC. Additionally, f-PVC contains a large percentage (typically from 10 to 40 percent (%)) of phthalate plasticizer. Such plasticizers have recently come under scrutiny because of medical and health concerns associated with migration of plasticizer from products that come into intimate contact with the human body, e.g., medical products, food products or toys, or because of leaching to the environment.
Flexible PVC film and sheet is used in many packaging, containment, decorative and protective applications that rely on the physical strength, flexibility, gas impermeability, low cost and HF sealability characteristics of the polymer. With the growing interest in replacing PVC with halogen-free polymers, much attention has been focused on polyolefin polymers such as polypropylene (PP), polyethylene (PE), metallocene polyethylene (mPE), styrenic-olefinic block copolymers and ethylene copolymers like ethylene-vinyl acetate (EVA). Although these polymers duplicate many f-PVC properties, none of them exhibit adequate dielectric properties to permit efficient HF sealability. While films or sheets made from these substitutes for PVC can be thermally welded or heat-sealed, they are not appropriate for HF activation in general or for RF sealing in particular.
Various halogen-free polymers have been described in the literature as exhibiting dielectric properties that permit HF or RF welding or sealing, e.g., thermoplastic polyurethane (TPU), polyamide (nylon) and glycol modified polyester (PETG). However, these polymers cost more than PVC, making direct substitution for f-PVC economically unattractive. In addition, some of the alternate RF active polymers have a significantly higher tensile modulus or stiffness than f-PVC, making the substitution in flexible film packaging or bag applications unfeasible.
Another approach to replace f-PVC with halogen-free polymers, uses copolymers of olefins with acrylate esters or vinyl acetate (VA). By copolymerizing higher levels (generally greater than ( greater than ) 15 percent by weight (wt %), based on copolymer weight) of VA or methyl acrylate with ethylene, some measure of RF activity can be achieved. While such olefin copolymers exhibit tensile and modulus properties similar to those of f-PVC and cost less than TPU, nylon and PETG, they have a dielectric loss factor (DLF) that is significantly lower than that of f-PVC. Consequently in RF sealing or welding operations, films made from copolymers of olefins with alkyl acrylates or VA require larger RF generators with a concomitant increase in both capital expenses and power usage, and longer welding times resulting in higher final part costs.
Another approach to incorporating RF activity into a halogen-free polymer is by blending in a RF active inorganic or organic particulate additive, typically at high loading levels. EP 193,902 discloses RF energy sensitized compositions in which inorganic sensitizers such as zinc oxide, bentonite clay, and alkaline earth metal aluminosilicates can be added at 1 to 20 wt % to a composition. WO 92/09415 describes incorporating RF receptors such as phosphonate compounds, phosphate compounds, quaternary ammonium salts, polystyrene sulfonate sodium salt, alkaline earth metal sulfate, and aluminum trihydrate into thermoset compounds and films. U.S. Pat. No. 5,627,223 discloses adding 1 to 50 wt % starch (to impart RF weldability) to a polyolefin blend that also contains a coupling agent. However, incorporation of inorganic or organic particulates will adversely affect film optics and clarity, tensile strength and toughness properties.
Several references teach that CO-containing ethylene copolymers exhibit excellent dielectric properties making them suitable for RF welding. For example, a series of USPs (U.S. Pat. Nos. 4,600,614; 4,601,865; 4,601,948; 4,660,354; 4,671,992; 4,678,713; 4,728,566; 4,766,035; 4,787,194; 4,847,155; and 4,895,457) teaches the use of CO-containing ethylene copolymers, e.g., ethylene-CO (ECO), ethylene-acrylic acid-CO (EAACO) and ethylene-vinyl acetate-CO (EVACO) for applications involving RF weldability and microwave heatability. With high levels of CO, CO-containing copolymers have excellent RF sealability and processability, but the polar nature of the copolymer results in lower interlayer cohesion with adjacent non-polar polyolefin layers in multilayer films. Conversely, with lower levels of CO, the RF activity is not sufficient to allow high speed RF sealing operations. U.S. Pat. No. 4,678,713, along with WO 86/07012, disclose coextruded multi-ply laminates in which at least one ply comprises a CO-containing polymer with RF sealability. Such laminates find use in the construction of multi-wall bags or containers. However, these disclosures are primarily concerned with coextruded multi-ply laminates in which at least one ply is a halopolymer.
WO 96/05056 teaches a thermoplastic polymer blend of a non-polar polyolefin (PO) and a polar ethylene copolymer having CO functionality. The blend contains from 1-90 wt % polar copolymer, based on blend weight. The blend forms a peelable seal layer for an easy opening package rather than a permanent seal. In general, seal strength decreases with increasing polar copolymer content.
EP 0703271 A1 discloses blends of EVA, very low density polyethylene (VLDPE) and, optionally, an EVACO terpolymer that are useful in providing flexible non-halogen containing thermoplastic polyolefin compositions for roof liners.
U.S. Pat. No. 5,029,059 discloses multilayer oriented, heat shrinkable thermoplastic films which may contain ECO copolymers. Halopolymers are recommended as preferred components and RF weldability is not mentioned.
In a first aspect, the present invention is a multilayer film comprising at least (a) a polar layer having a dielectric loss factor of at least (xe2x89xa7) 0.10 comprising an ethylene copolymer with carbon monoxide (CO) wherein the CO comprises at least 3 percent by weight of the polar layer and (b) a layer comprising a non-polar olefin homopolymer or a non-polar olefin copolymer. The films of the present invention are free of halogen containing polymer and exhibit HF or RF sealability in high speed manufacturing operations.
In a second aspect, the present invention is a halogen-free, HF sealable film comprising a blend of at least two olefin polymers, wherein one olefin polymer is a non-polar homopolymer or a non-polar copolymer and at least one olefin polymer has polymerized therein at least ethylene and CO, the CO being present in an amount sufficient to give the blend a DLF of at least 0.10. The amount of CO is desirably xe2x89xa73 wt %, based on total blend weight. The film is preferably a monolayer film, more preferably a substantially phthalate plasticizer-free film. The film of this aspect can, however, function as the polar layer of the multilayer film of the first aspect.
A third aspect of the present invention is an article of manufacture fabricated from the film of either the first aspect or the second aspect. The article of manufacture desirably includes at least one segment wherein the film is sealed to itself, a substrate or both at a seal interface. Sealing-preferably results from exposure of the film to HF or RF energy. The seal interface preferably has a bond strength of at least one pound per inch (lb/in) (0.18 Newton per millimeter (N/mm)).
Such films, which can replace f-PVC without the use of plasticizers and which are RF sealable, are particularly suitable for applications in which the film, or the products in contact with the film, come into intimate contact with the human body. Such applications may include medical or urological collection bags, medical ostomy bags, medical infusion or intravenous (IV) bags, inflatable devices such as air mattresses, flotation devices or toys, food packaging, retail product blister packaging, children""s articles and toys, reinforced laminates for tents and tarpaulins, roofing membranes and geotextiles, and stationery applications such as notebook binder covers. Compositions that yield the films of the present invention can also be extruded into a tubing with an RF active outer layer. Such tubing can readily be used in conjunction with RF weldable films to provide a complete RF welded PO film structure such as a medical collection bag. Skilled artisans can easily expand this illustrative listing to include virtually any device or application that requires an HF or RF sealable, flexible, mono-layer or multilayer film structure. The relatively low (compared to f-PVC) cost of PO materials used to make the films of the present invention and the performance features of such films opens many opportunities for replacement of flexible, plasticized, halogenated films such as f-PVC.
Unless otherwise stated, a range includes both endpoints used to state the range.
xe2x80x9cHalogen-freexe2x80x9d, as applied to films of the present invention, refers to polymer materials used to form the films. The polymer materials lack chemically combined halogen atoms. In other words, halogenated monomers do not constitute building blocks for the polymer materials. In addition, the polymers are not halogenated subsequent to formation as in the case of chlorinated polyethylene prepared via a slurry chlorinating process. The films may, however, contain small amounts of non-polymeric halogenated additives, including conventional halogenated fire retardant additives.
DLF is a calculated value determined by multiplying a material""s dielectric constant (DC) by its dielectric dissipation factor (DDF) (or loss tangent). The DC and DDF are readily determined by instrumented dielectric testing methods. An especially preferred test fixture uses a Hewlett-Packard Impedance/Material Analyzer, Model 4291B coupled with a Hewlett-Packard Dielectric Test Fixture, Model 16453A. Dielectric properties can be measured on compression molded plaques (diameter of 2.5 inches (in) (64 millimeter (mm)) and a thickness of 0.050 in (1.3 mm) formed from a material such as a polymer or a blended polymer compound.
xe2x80x9cHF sealabilityxe2x80x9d refers to the bonding of a sealable polymer to a portion of itself or to another material using electromagnetic energy frequencies of 0.1-30,000 megahertz (MHz). This includes RF heating and microwave (MW) heating rather than conventional heat sealing. The HF range generically covers three frequency ranges more commonly referred to as an ultrasonic frequency range (18 kilohertz (KHz)-1000 KHz), the RF range (1 MHz-300 MHz), and the MW frequency range (300 MHz-10,000 MHz). The RF and MW ranges are of particular interest. The terms xe2x80x9cactivatingxe2x80x9d, xe2x80x9csealingxe2x80x9d, xe2x80x9cbondingxe2x80x9d, and xe2x80x9cweldingxe2x80x9d (and variations of each word) are used interchangeably herein.
In general, skilled artisans regard a material with a DLF of less than ( less than )0.05 as RF or HF inactive. They classify materials with a DLF within a range of 0.05-0.1 as weakly RF or HF active. They consider materials with a DLF  greater than 0.1 to have good RF or HF activity, and materials with a DLF  greater than 0.2 to be very RF or HF active, and thus exhibits excellent RF sealability. While a DLF of 0.1 may produce satisfactory results, skilled artisans typically prefer a DLF  greater than 0.1, more often  greater than 0.15 and still more often greater than 0.2, in order to obtain sufficient sealing by application of HF waves in general and RF waves in particular.
An xe2x80x9colefin polymer having polymerized therein at least ethylene and COxe2x80x9d and an xe2x80x9cethylene copolymer with COxe2x80x9d. both generically refer to polymers prepared by polymerizing CO with ethylene and, optionally, one or more monomers that have ethylenic (olefinic) unsaturation using a conventional catalyst (such as a Ziegler-Natta catalyst), a metallocene catalyst (including constrained geometry catalysts), or both. The latter monomers include those containing 3 to 20 carbon atoms (C3-20), especially C3-8 alpha-olefin (xcex1-olefin) monomers. The polymers may also include one or more of C3-8 unsaturated organic acids, such as acrylic acid, methacrylic acid and 1-butenoic acid, alkyl esters or metal salts of these acids, such as ethyl acrylate, methyl methacrylate, n-butyl acrylate, sodium acrylate and potassium methacrylate, and vinyl acetate. The polymers must contain an amount of CO sufficient to render the polymer susceptible to heating by HF radiation in general and RF radiation in particular. This amount should provide a DLF xe2x89xa70.1, preferably xe2x89xa70.15. The amount of CO desirably exceeds 3 wt %, based on total polymer weight. The amount preferably lies within a range of from 4 wt % to  less than 50 wt %, based on total polymer weight, more preferably within a range of from 6 to 40 wt %, still more preferably within a range of from 8 to 30 wt %.
Preferred olefin polymers having polymerized therein both ethylene and CO include ECO copolymers, EVACO terpolymers, EAACO terpolymers and ethylene-n-butyl acrylate-CO (EnBACO). Many skilled artisans interchangeably use xe2x80x9ccopolymerxe2x80x9d and xe2x80x9cinterpolymerxe2x80x9d to refer to polymers having polymerized therein at least two monomers. While adopting that convention, the above illustrations use copolymers to refer to the presence of two polymerized monomers and terpolymers to refer to the presence of three polymerized monomers. In that context, four polymerized monomers could be called a tetrapolymer, but is more often referred to as an interpolymer.
While the polar layer of the multilayer film of the first aspect may entirely comprise an ethylene copolymer with CO, the polar layer preferably comprises a blend of an ethylene copolymer with CO with a non-polar olefin homopolymer, a non-polar olefin copolymer or both. The non-polar polymers lack a detectable CO content (based on current analytical technology) and are sometimes referred to herein as xe2x80x9cCO-freexe2x80x9d. As such, the polar layer, when it contains a non-polar homopolymer, a non-polar copolymer or both, preferably has a composition equivalent, if not identical, to that used to make the films of the second aspect of the present invention. The only requirement is that the overall CO content of the polymer blend be xe2x89xa73 wt %, based on weight of the polymer blend. Preferably, the ethylene copolymer with CO comprises from about 30 to about 80 wt % of the polymer blend.
Suitable non-polar polymers include any thermoplastic olefin polymer other than an xe2x80x9cethylene copolymer with COxe2x80x9d as defined herein. The non-polar olefin polymer may be a homopolymer, such as PE or PP, or a copolymer such as ethylene-butene-1 (EB), ethylene-octene-1 (EO) or ethylene-propylene (EP). Useful non-polar olefin polymers include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), high density polyethylene (HDPE), polyethylene plastomer (metallocene catalyst, 0.86-0.92 grams per cubic centimeter (g/cc) density (mPE)), polypropylene homopolymer (PP), polypropylene copolymer (co-PP), EVA, ethylene-methyl acrylate (EMA), ethylene-n-butyl acrylate (EnBA), ethylene-ethyl acrylate (EEA), ethylene-acrylic acid (EAA), ethylene-methacrylic acid (EMAA), ionomerized metal salts of carboxylic acid copolymers, such as sodium, potassium or zinc ionomers of EAA or EMAA, ethylene-propylene-diene terpolymer, (EPDM), ethylene-styrene interpolymer (ESI), ethylene-vinyl alcohol (EVOH), polybutene (PB), polyisobutene (PIB), styrene-butadiene (SB) block copolymer, styrene-isoprene-styrene (SIS) block copolymer, styrene-ethylene-butene-styrene (SEBS) block copolymer or maleic anhydride (MAH) grafted (g) POs such as EVA-g-MAH, PE-g-MAH and PP-g-MAH. Preferred non-polar olefin polymers include at least one of LDPE, LLDPE, mPE, PP, co-PP, ESI, EAA, EVA, EMA, and MAH-g-POs. The polymer blends typically contain from 20 to 70 wt % non-polar olefin polymer(s), based on blend weight, but greater or lesser amounts may be used if the blend has an overall CO content of xe2x89xa73 wt %.
The films of the present invention may, and preferably do, further comprise a compatibilizer, typically an additional olefin polymer. Suitable compatibilizers include ethylene copolymers having polymerized therein, or grafted thereto, one or more polar comonomers. The compatibilizers promote at least macroscopic or visual blend uniformity between a polar CO-containing polymer and a non-polar olefin polymer. Illustrative compatibilizers include EVA, EnBA, EMA, EEA, EAA, olefins grafted with dicarboxylic acid or anhydride, styrenic block copolymers and ESI. These compatibilizers can be utilized in the polymer blend used to make the films of the second aspect or in layer (a), layer (b), or both layers (a) and (b) of the multilayer films of the first aspect of the invention.
The polymer blends that form the films of the present invention may also include one or more conventional additives that impart a functional attribute to the films, but do not significantly detract from film sealability via exposure to HF or RF irradiation. Such additives include, without limitation, antioxidants or process stabilizers, ultraviolet (UV) stabilizers, tackifiers, fire retardants, inorganic fillers, biocides, and pigments.
The films described herein may be of any gauge that serves a given need, but typically have an overall gauge within a range of from 0.5 to 100 mils (12 to 2500 micrometers (xcexcm)), preferably from 1 to 40 mils (25 to 1000 xcexcm) and most preferably from 2 to 20 mils (50 to 500 xcexcm).
Any conventional film forming process may be used to fabricate films of the present invention. Illustrative processes suitable for use in making films of the second aspect include, without limitation, an annular extruded blown film process, a slot die cast extrusion film process, and extrusion coating of one or more layers upon a film or substrate. The multilayer films of the first aspect of the present invention have xe2x89xa7two layers, with xe2x89xa7one CO-containing polar layer and xe2x89xa7one adjacent layer comprising an olefin homopolymer or copolymer. Such multilayer films can be produced by a conventional annular coextruded blown film process, a slot die cast coextrusion film process, extrusion coating of multiple layers upon a film or substrate, or lamination of multiple plies of film layers. Additionally, the polymer compositions disclosed can be dissolved in solvent or dispersed as an aqueous dispersion or emulsion and coated from a liquid phase using conventional liquid coating processes. In addition, the films of the present invention can be fabricated into extruded profile shapes such as tubing. For example, a RF-weldable monolayer or coextruded, multi-layer, tubular structure may be bonded to a film or other substrate to fabricate a composite part such as a medical collection bag. The polymer blend compositions described herein can also be dissolved in solvent or dispersed as an aqueous dispersion or emulsion and coated from a liquid phase using conventional liquid coating processes.
In order to achieve RF weldability of a multilayer film of the first aspect of the present invention to itself or to another substrate, the CO-containing RF active layer must either be the surface bonding or adhesive layer or it must be in close proximity to the surface bonding or adhesive layer such that the heat generated by the RF activation of the CO-containing layer can quickly transmit to and through the surface bonding or adhesive layer, melting this layer and causing thermal bonding and sealing.
For multilayer films in which the CO-containing RF active layer is a surface bonding or adhesive sealing layer, the layer is desirably xe2x89xa710% of the overall film gauge, and preferably xe2x89xa720% of the overall film gauge for films  less than 10 mils (250 xcexcm) in thickness. For films thicker than 10 mils in which the CO-containing RF active layer is a surface bonding or adhesive sealing layer, the minimum layer thickness must be xe2x89xa71.0 mil (25 xcexcm).
For multilayer films in which the CO-containing RF active layer is not a surface bonding or adhesive sealing layer, and is covered by an adjacent adhesive sealing layer, the layer thickness of the sealing layer should be kept to a minimum so as not to inhibit heat transfer and polymer melting of this seal layer after the adjacent RF active layer is activated and heated by the imposed RF field. For multilayer films thinner than 10 mils (250 xcexcm), the CO-containing RF active layer should be xe2x89xa720% of the overall film gauge, and preferably xe2x89xa730% of the overall film gauge so as to provide sufficient heat generation to melt the adjacent adhesive sealing layer and cause bonding to occur.
Any of the films described herein can be sealed or welded to itself or to another substrate using a conventional HF sealer, such as a RF sealer. Commercially available RF welders, such as those available from Callanan Company, Weldan, Colpitt, Kiefel Technologies, Thermatron, Radyne and others, typically operate at a frequency of 27.12 MHz. Two less frequently used RFs are 13.56 MHz and 40.68 MHz. Typical MW sealing or welding apparatus function at frequencies of 2450 MHz (2.45 GHz), 5800 MHz (5.8 GHz) and 24.12 GHz.
RF or MW activation (sealing and bonding) offers a performance advantage over conventional thermal or heat sealing when rapid sealing time becomes a dominant factor, such as in high speed manufacturing. HF (including RF and MW) bonding technologies allow energy to be concentrated at the HF active layer, thus eliminating the need to transfer heat through an entire structure. This is particularly advantageous with thick films (gauge  greater than 5 mils (125 xcexcm)) where conventional thermal sealing would require relatively long contact times to permit thermal transfer through the thick polymer structure to the bonding interface. RF seal times as short as 0.4 second can be used wherein conventional thermal contact or impulse sealing might require a several seconds (or longer) seal time with thicker films. HF bonding is also advantageous when a thermally sensitive material is used within the composite, such as a color sensitive dyed fabric or nonwoven or an oriented film which can soften and undesirably shrink upon heating. RF dies can also be fabricated in very complex shapes, which is difficult to do with thermal sealing equipment.
The films of the present invention facilitate fabrication of a variety of structures via HF sealing. For example, a film can be folded over and at least partially HF sealed to itself to form a bag or a pouch. Two plies of the same film readily form a bag or pouch without a fold. HF sealing also promotes bonding of such a film to a substrate such as another film, nonwoven fabric, injection molded or extruded parts, or paper. For most applications, sufficient HF sealing or bonding equates to an adhesive strength of xe2x89xa71.0 pound per inch (lb/in) (0.18 Newton per millimeter (N/mm)). In the case of RF welding of medical collection bags or drainage pouches, the RF weld between the two plies of film must be strong enough that the films cannot be peeled apart without tearing the film or exceeding the strength of the film itself. This requires seal strengths of xe2x89xa74.0 lb/in (0.70 N/mm), as tested by the 180 degree peel test of ASTM D-903. Thicker film structures, such as those used for inflatable applications, generally require even greater weld or bond strengths. The film adhesive or seal layer must be formulated in conjunction with the CO-containing RF active layer so as to achieve a permanent bond rather than an unacceptable peelable seal. Films like those of the present invention, but with a total CO content  less than 3 wt %, typically yield peelable seals that fail the above adhesive strength requirements when exposed to the same level of HF radiation. Similarly, interlayer adhesion between the coextruded layers of the film must be high enough that the layers intimately bond together and do not peel apart at less than desired seal strengths. As such, compositions of the various layers must be formulated so that the layers will cohesively bond together.
Notwithstanding emphasis upon HF weldability, films of the present composition can also be thermally laminated, sealed or welded using conventional thermal processes such as hot roll lamination, flame lamination, and heat sealing. With this capability, one can combine a thermal process with HF welding. One illustration of such a combination involves a first step of thermally laminating a film of the present invention to a substrate such as a fabric thereby forming a film/fabric composite and a second, sequential step of HF welding two composites together at a film/film interface, thereby providing film interior surfaces and fabric exterior surfaces.